Methanol-to-olefin processes are well described in the art. Typically, methanol-to-olefin processes are used to produce predominantly ethylene and propylene. An example of such a methanol-to-olefin process is described in WO-A 2006/020083. In the process of WO-A 2006/020083, the methanol is first converted into dimethylether (DME) prior to be subjected to a conversion to olefins, thereby reducing the amount of water produced during the conversion to olefins. Both methanol and DME are suitable feedstocks for a Methanol-to-olefin process and therefore such processes are also generally referred to as oxygenate-to-olefin (OTO) processes.
In EP2024303A1, another OTO process is described wherein in addition to oxygenates, also C4 and C5 olefins are provided to the OTO process. These olefins are provided as an olefinic co-feed together with the oxygenates. By providing an olefinic co-feed to the OTO process more ethylene and propylene may be produced. According to EP2024303A1, suitable sources for these olefins are for instance C4 and C5 hydrocarbon fractions obtained from refinery units such as thermal cracking units, catalytic cracking units, steam cracking units, naphtha (steam) cracking units, butadiene extraction units, semi-hydrogenation units for removal of diolefins. Another suitable source for C4 and C5 olefins, disclosed in EP2024303A1, are C4 and C5 olefins which are retrieved from the effluent of the OTO reaction zone. According to EP2024303A1, these C4 and C5 olefins are suitably provided back to the OTO reaction zone as part of a recycle stream to become part of the olefinic co-feed.
However, both the externally provided hydrocarbon fractions as well as the internal recycle stream typically contain paraffinic hydrocarbons which are undesired in the feed to an OTO reaction zone. Paraffins are not converted in the OTO reaction zone and accumulate in the internal recycle. To prevent undesired high levels of paraffins accumulating in the recycle stream, typically part of the recycle stream is purged from the process. However, as part of this purge stream, also valuable olefins are purged from the process.
There is a need in the art to make optimal use of the available olefins to produce ethylene and propylene, while reducing the amount of paraffins provided to the OTO reaction in an OTO process.